Separately-formed innersoles are often loosely inserted into athletic and other shoes. Some of these innersoles are thermoformed to the general shape of the foot and shoe, for better foot support. Others are simply flat sheets cut to shape. Sometimes a permanently-deformable material (i.e., one with a high compression set) is used to permanently conform the innersole to the user's foot during initial use. Sometimes a cushioning material (i.e., one with low compression set) is used to improve comfort.
Various materials and combinations of materials have been used to provide these properties. Microcell polyurethane foam, which exhibits a very low compression set (less than 10%), has been used to make flat, nonthermoformed innersoles with good cushioning (examples are those sold under the Dr. Scholl's trademark). Polyethylene foam has been used in thermoformed innersoles. Non-microcell polyurethane foam exhibiting a high compression set has been laminated with polyethylene foam to form a two-layer innersole in which the polyurethane layer has a high compression set to provide permanent deformation and in which the polyethylene layer is thicker than the polyurethane and provides cushioning.